“Normal” status of a switch
The “normal” status for a switch is the status its electrical
Listing : “normal” definitions various discrete sensor types
Hand switch: no one pressing the switch
Limit switch: target not contacting the
switch
Proximity switch: target far away
Pressure switch: low pressure (or even a
vacuum)
Level switch: low level (empty)
Temperature switch: low temperature
(cold)
Exp. two pressure switches sensing the same
fluid pressure
Mercury tilt switches
Level switches
metal tuning fork structure
capacitive switches
4 to 20 mA analog current signals
Typically, a 4
milliamp current
value represents 0% of scale, a 20
milliamp current value represents 100% of scale.
example, a 4-20 mA
Graphical interpretation of signal ranges
“manipulated variable”
Most industrial control
systems use at least two
different 4-20 mA signals: one to represent the process
variable (PV) and one to
represent the command signal to the final control element (the“manipulated variable” or MV)
Typically 4 milliamps
commands a closed
(shut) control valve or a stopped motor,
while 20 milliamps commands a
